The first four years of life is the period of most rapid brain growth and a high-risk time for brain tumors. The intent of these studies is to examine the relationship between brain growth and brain tumor formation. Elucidating how pediatric brain tumors take advantage of the microenvironment of the growing brain to stimulate their growth will advance our understanding of brain tumor biology and constitute a new paradigm for research into novel anti-tumor agents. CXCL12 and its receptor, CXCR4, constitute an important regulatory pathway in normal brain development and in brain tumor growth. CXCL12's growth promoting activity is tightly regulated during normal brain development but unregulated in tumor cells. In published studies CXCR4, a G?i coupled receptor, stimulated brain tumor growth via sustained reductions in intracellular cAMP. Ordinarily, sustained responses to G protein coupled receptors are prevented by a negative feedback mechanism termed desensitization, which involves phosphorylation of the receptor by G protein receptor kinases. This process was deficient in tumor cells where sustained CXCL12-induced cAMP suppression was associated with a lack of CXCR4 phosphorylation. This proposal addresses the hypothesis that CXCR4 desensitization is a molecular switch that regulates its growth effects: That, 1) CXCR4 desensitization must be inhibited in order for CXCR4 to stimulate growth, 2) that sonic hedgehog inhibits CXCR4 desensitization during normal cerebellar development and 3) that this normal developmental pathway is co-opted in medulloblastoma where desensitization is disabled. Three parallel experimental systems are utilized that encompass a continuum from normal brain development to malignant brain tumor growth, to determine the relationship between CXCR4 desensitization and growth effects. 1) In cerebellar tissue and primary cultures of cerebellar granule neurons expressing wildtype or phosphorylation mutant forms of CXCR4, the regulation of CXCR4 desensitization during development and by Shh will be measured. 2) G protein receptor kinase expression, CXCR4 phosphorylation, and desensitization will be directly measured in primary cultures of medulloblastoma and other brain tumors. 3) The effect of altered GRK expression and CXCR4 desensitization on intracranial growth and xenograft responses to anti-tumor agents will be determined in a novel intracranial primary human brain tumor xenograft system.